Often, a person will post a sincere message about which he/she is emotionally sensitive. Skillful trolls know that an easy way to upset the person is to falsely claim that they are a "troll." In forums where most users are similar to each other, outsiders may be perceived as trolls simply because they do not fit into the social norms of that group.
(wikipedia)
So, it seems unamerican behaviour is denounced as trolling?